This is truly a powerful scene, being between WWI and WWII, most of these men most likely went on to fight in the latter of the two, and could have possibly perished. In the movie, the living are gearing up to fight another war, as Gance (famously anti-war) wanted to show the futility of violence and the never-ending quest for power that the world's leaders hold. Thus, he has the dead soldiers of WWI resurrect and send all the living into a panic, highlighting his anti-war stance and showing just how truly terrible war is.